Hafiz Sakhawi (may Allah have mercy on him) writes:

«اجْتَنِبْ أَيُّهَا الْكَاتِبُ الرَّمْزَ لَهَا أَي لِلصَّلَاةِ عَلَى رَسُولِ اللَّهِ صَلَّى الله عَلَيْهِ وَسَلَّمَ فِي خَطِكَ، بأن تَقْتَصِرَ مِنْهَا عَلَى حَرْفَيْنِ، وَنَحْوِ ذلِكَ، فَتَكُونَ مَنْقُوصَةٌ صُورَةٌ، كَمَا يَفْعَلُهُ الكُسَالَى وَالْجَهَلَةُ مِنْ أَبْنَاءِ الْعَجَمِ غَالِبًا وَعَوَامُ الطَّلَبَةِ، فَيَكْتُبُونَ بَدَلًا عَنْ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : ص ، أَوْ صم، أَوْ صلم، أَوْ صلعم، فَذَلِكَ لِمَا فِيهِ مِنْ نَّقْصِ الأَجْرِ لِنَقْصِ الكِتابَةِ خِلافُ الأولى»

"O writer! Avoid writing abbreviations for sending blessings (Durood) upon the Messenger of Allah (peace and blessings of Allah be upon him) in your handwriting by restricting it to two, three, or four letters. In this way, the form of the blessing becomes deficient, just as is the practice of the lazy and many ignorant non-Arabs (Ajamis), and the generality of students also fall into this mistake. They write 'S', 'Sm', 'Slm', or 'Sl'am' instead of 'peace and blessings of Allah be upon him'. This method is undesirable due to the reduction in reward caused by the deficiency in writing."

[Fath al-Mugheeth bi-Sharh Alfiyyat al-Hadith: 3/71-72]